Methanol, known as one of the basic organic chemicals, is essentially manufactured from mixtures of hydrogen with carbon oxides, currently called "synthesis gas", according to the balanced reactions: EQU CO+2H.sub.2 .revreaction.CH.sub.3 OH (1) EQU CO.sub.2 +3.sub.2 .revreaction.CH.sub.3 OH+H.sub.2 O (2)
Almost all the synthesis gas used for manufacturing methanol is obtained by steam-reforming of natural gas, whose main component is methane and which reacts with water according to the following reactions: EQU CH.sub.4 +H.sub.2 O.revreaction.CO+3H.sub.2 ( 3) EQU CH.sub.4 +2H.sub.2 O.revreaction.CO.sub.2 +4H.sub.2 ( 4)
Methane homologs, present in minor proportion in natural gas, react similarly accoring to a well-known stoichiometry.
The leading role of natural gas as raw material in methanol synthesis is justified, economically by its large availability and its attractive price, and technically by its ease of use and the high purity of the synthesis gas obtained therefrom.
The use of natural gas suffers however from a disadvantage: the composition of the synthesis gas obtained therefrom is far from being the optimum composition required for methanol synthesis.
As a matter of fact, when considering equations (1) and (2) in comparison with equations (3) and (4), it is apparent that, for each mole of methanol manufactured according to (1) and (2), the reforming gas obtained according to (3) and (4) gives one hydrogen molecule in excess.
Even in the favorable cases where it can be upgraded, said hydrogen is always produced at a cost which exceeds that of hydrogen produced by more suitable methods.
Another disadvantage of the reforming gas is its low pressure as compared to the requirements for methanol synthesis, both for technological and thermodynamic reasons.
As a matter of fact, at constant temperature and H.sub.2 O/C ratio, the higher the pressure, the larger the unconverted methane proportion in the synthesis gas (P. Wellman and S. Katell, Hydrocarbon Processing, No 6, vol 42, June 1962).
An increase of the reaction temperature might theoretically increase the methane conversion, but an upper limit is imposed by the metal flow hazards, and it is hence difficult to operate at temperatures higher than 850.degree. C. in reforming tubes.
Similarly, for reasons of power consumption, the H.sub.2 O/C ratio cannot be increased beyond 3-4.
With these limitations and in order to avoid an excessive increase of the residual methane content, steam-reforming of natural gas is usually performed at a pressure not exceeding 3 MPa, whereas methanol synthesis usually requires pressures of about 5-12 MPa.
Another method known for producing a synthesis gas is the oxidation of methane by oxygen. From the strictly chemical point of view, the ideal reaction for manufacturing synthesis gas from natural gas is the following: EQU CH.sub.4 +1/2O.sub.2 .revreaction.CO+2H.sub.2 ( 5)
This reacton is currently performed in partial oxidation catalytic processes such as ONIA-GEGI process, for example.
The major disadvantage of these catalytic processes results from their operation at atmospheric pressure.
The compression cost for increasing their pressure to the level required for methanol synthesis makes them economically unsatisfactory.
In the middle of the fifties, attempts were made to operate these catalytic processes under pressure. The results were disappointing since every time the gas was admixed with oxygen under high pressure and at high temperature explosion phenomena and clogging of the catalyst bed by excessive soot formation occurred, thus requiring the termination of the experiments (Du Bois Eastman, Ind. Eng. Chem. vol 48, p. 1118-1122, July 1956).
For this reason, the present partial oxidation processes, such as the Shell process (C. I. Reed and C. J. Kuhrese, Hydrocarbon Processing vol 67 (9), p. 191-194, September 1979) or the Texaco process (W. L. Slater and R. M. Dille, Chem.Eng.Prog. vol 61 (No 11), p. 85-88, November 1965) are all operated under pressure but without catalyst and with O.sub.2 /C ratios of at least 0.7 oxygen mole per carbon atom contained in the hydrocarbons.
By these processes, synthesis gas can be obtained under sufficient pressure for direct methanol synthesis, but with the disadvantage of a large carbon monoxide excess in proportion to hydrogen.
Sometimes, the CO in excess may be used to manufacture acetic acid. However the disproportion between the methanol market and that of acetic acid does not permit one to rely on such a favorable CO upgrading.
Also, so-called primary and secondary reforming units can be used wherein air, optionally enriched with oxygen, is introduced at the output of the conventional steam-reforming, into the cracked gas, the resultant effluent being subjected to a so-called secondary catalytic reforming step (D. R. Holland and S. W. Wan, Chem.Eng.Prog. vol 59 (8), p. 69-74, August 1963).
These units cope with the difficulties indicated by Du Bois Eastmann since, as a result of the dilution with nitrogen and of the small percentage of introduced oxygen, the mixture is clearly below the lower explosive limit.
In view of the presence of nitrogen and of the high H.sub.2 /CO ratio, these units can only be used to prepare synthesis gas for ammonia.
As a matter of fact, in this type of use, nitrogen, instead of being an expensive inert diluent is, on the contrary, an indispensable raw material.
Recently, by removing nitrogen and increasing the oxygen proportion, it has been proposed (French Pat. No. 2372116) to extend this operating mode to pure oxygen and to the manufacture of gas for synthesis methanol.
As shown in Table I below, the range of explosiveness of carbon monoxide and hydrogen is even larger than that of methane and the proposed improvement does not avoid the difficulties indicated by Texaco Company.
TABLE I ______________________________________ FLAMMABILITY LIMITS OF GASES AND VAPORS IN OXYGEN (According to Patty) Flammability limits (in vol %) Lower Higher ______________________________________ Methane CH.sub.4 5.15 60.5 Ethane C.sub.2 H.sub.6 3.05 66.0 Hydrogen H.sub.2 4.65 93.9 Carbon monoxide CO 15.5 93.9 ______________________________________